Side Seal for Wet Lens Elements

ABSTRACT

A method for protecting a wet lens element from liquid degradation is provided. The method includes applying a thin coating of an organoxy-metallic compound to the side portions of a wet lens element to leave behind an optically inert, light absorbing metal oxide film. A liquid shield coating is applied on top of the metal oxide coating. The two coating layers protect the wet lens element from liquid degradation when the side portion of the wet lens element is submerged into a liquid. In an embodiment, the wet lens element is an immersion lithography wet lens element and the liquid is an immersion lithography liquid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/473,068, filed on Jun. 23, 2006, which is a continuation-in-part ofU.S. patent application Ser. No. 10/253,655, filed on Sep. 25, 2002,(now U.S. Pat. No. 7,081,278) both of which are incorporated byreference herein in their entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to immersion lithography, moreparticularly, to providing side seals for a wet lens element to protectthe wet lens element from degradation.

2. Related Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays (e.g., liquid crystal displays), semiconductorwafers, circuit boards, various integrated circuits, print heads,macro/nano-fluidic substrates, and the like. During lithography, asubstrate, which is disposed on a substrate stage, is exposed to animage projected onto the surface of the substrate by exposure opticslocated within a lithography apparatus.

The projected image produces changes in the characteristics of a layer,for example, photoresist, deposited on the surface of the substrate.These changes correspond to the features projected onto the substrateduring exposure. Subsequent to exposure, the layer can be etched orotherwise processed to produce a patterned layer. The patterncorresponds to those features projected onto the substrate duringexposure. The patterned layer is then used to remove or further processexposed portions of underlying structural layers within the substrate,such as conductive, semiconductive, or insulative layers. This processis repeated, together with other steps, until the desired features havebeen formed on the surface, or in various layers, of the substrate.

In the field of immersion lithography the exposure operation isconducted with an immersion liquid, which is typically water, betweenthe last lens element of the projection lens and the substrate. Thisrequires certain design modifications to the apparatus. A number of suchlithographic apparatus use calcium fluoride (CaF₂) lenses as the lastelement in the projection lens. This element is therefore constantly incontact with the immersion liquid. To achieve the required opticalproperties such immersion lithography apparatus often use ultra purewater (“UPW”). Unfortunately, calcium fluoride dissolves in ultra purewater and the lens needs to be protected from the water in order toprevent surface damage that can lead to unwanted scattering of light.For example, when calcium fluoride dissolves into the immersion liquid,some of the calcium fluoride will crystallize onto a work surface, suchas a wafer, from microdroplets that are left behind. These microdropletscan lead to imperfections in the printed image on a wafer or other worksurface.

Non-optical areas of the lens also dissolve calcium flouride into thewater. Due to the water conditions, the rate of dissolution far exceedsthe purity specifications of the water. Consequently, the purity of thewater (or other liquid) is affected which leads to defects to wafers orother surfaces. It is therefore necessary to protect both the opticalsurfaces of a lens and the non-optical sides of the lens that come incontact with the water.

What is needed is a method for protecting the non-optical sides of a wetlens element from degradation which are immersed in a liquid.

SUMMARY

The present invention is directed to a method for protecting a wet lenselement from liquid degradation. The method includes applying a thincoating of an organoxy-metallic compound to the side portions of a wetlens element to leave behind an optically inert, light absorbing metaloxide film. A liquid shield coating is applied on top of the metal oxidecoating. The two coating layers protect the wet lens element from liquiddegradation when the side portion of the wet lens element is submergedinto a liquid. In an embodiment, the wet lens element is an immersionlithography wet lens element and the liquid is an immersion lithographyliquid.

Treatment of a wet lens element using the current invention providesseveral benefits. First, the combination of a liquid shield coating andthe metal oxide coating prevent the side portions of a wet lens elementfrom significant degradation when immersed in a liquid.

Second, applying the metal oxide coating to an optical element using thecurrent invention reduces the light-induced deterioration of a liquidshield coating used to protect the side portions of a wet lens elementfrom liquid degradation. Ordinarily, ultra-violet light will beinternally scattered and propagated to points on an optical elementwhere a liquid shield has been applied. The light emitted will impingeupon the liquid shield resulting in deterioration of the bond betweenthe liquid shield and the lens. Ultimately, deterioration can result inthe liquid shield falling off of the lens. The present invention willprevent the liquid shield deterioration and resulting undesirableaffects. The coatings applied also have the advantage of being robust tohandling and cleaning of the wet lens element.

Moreover, there are several advantages of the present invention overother methods that might be adapted to apply a coating to preventdegradation of a wet lens element. There are numerous types of systemsto apply thin coatings to polished surfaces of optical elements. Thesesystems are very expensive, require extensive set-up time, and, inparticular, do not provide a cost-effective way to coat the edge of anoptical element. Their design limits their ability to apply a thincoating to the edge of an optical element. Retrofitting or adaptingthese systems to apply a coating to an edge is time-consuming andcostly, and often, cost prohibitive. The application steps of thepresent invention do not require high precision, and thus do not requireexpensive application apparatus.

Even assuming that existing systems could be modified to apply a thincoating to the side portions of wet lens elements, the present inventionoffers additional advantages. Existing systems used to apply thincoatings to polished optical surfaces are designed to apply a coatingwith a very precise thickness. Such precision is not necessary whenapplying a coating to an optical element to prevent degradation of theside portion of a wet lense element when the optical element is placedin an immersion liquid. Thus, using modified current coating systemswould be more expensive and time consuming than the use of the currentinvention because of the system setup time needed and the additionalcomplexities associated with working with systems designed to be veryprecise.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention are described in detail below with reference toaccompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1 is a block diagram of an immersion lithography system.

FIG. 2A is a side view of a mechanical side seal for a wet lens element,according to an embodiment of the invention.

FIG. 2B is a cross sectional view of a mechanical side seal, accordingto an embodiment of the invention.

FIG. 3 is a flowchart diagram that shows a method for protecting a wetlens element from liquid degradation, according to an embodiment of thepresent invention.

FIG. 4A is a diagram of a side view of a wet lens element having a metaloxide and liquid shield coating, according to an embodiment of thepresent invention.

FIG. 4B is a diagram of a top-down view of a wet lens element having ametal oxide and liquid shield coating, according to an embodiment of thepresent invention.

FIG. 5 is a plot of test results showing wet lens element degradation inan immersion liquid.

DETAILED DESCRIPTION

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those skilled inthe art with access to the teachings provided herein will recognizeadditional modifications, applications, and embodiments within the scopethereof and additional fields in which the present invention would be ofsignificant utility.

In immersion lithography systems, liquid is injected into the spacebetween the projection optical system (“POS”) exit window and thesubstrate surface. FIG. 1 is a block diagram of a typical immersionlithography system 100. System 100 includes a pattern generator 102, aPOS 104, and a substrate 106. In order to completely expose substrate106, substrate 106 moves relative to POS 104. Immersion liquid 108 fillsthe space between substrate 106 and an exit POS element or wet lenselement 110.

FIG. 2A illustrates a side view of a mechanical side seal 210 for a wetlens element 110, according to an embodiment of the invention. This isone possible approach for preventing deterioration of the sides of alens that are in contact with an immersion liquid. FIG. 2B illustrates across sectional view of mechanical side seal 210, according to anembodiment of the invention. Mechanical side seal 210 is a thin sheet ofnon-polluting material (e.g., quartz, fluoroelastomer, or metal) thatwraps around the non-optical areas of wet lens element 110 to form amechanical shield between liquid 108 and wet lens element 110. Asdiscussed above, wet lens element 110 can be made of CaF₂. Mechanicalside seal 210 can either be connected to lens mount 205, as shown inFIG. 2A, or to wet lens element 110 outside the wetted areas. At theinterface between the optical areas, such as optical surface 225 andnon-optical or side areas 230 of the lens, a small gap 220, typicallythe average of which is around 150 microns, is provided to eliminatepossible deformations of optical surface 225.

Liquid will penetrate gap 220, and fill the volume between mechanicalside seal 210 and wet lens element 110. As a result, CaF₂ from wet lenselement 102 will still dissolve into the thin film volume of waterbetween side seal 210 and wet lens element 110. However, the amount ofarea that is able to exchange Ca or F into the ultra pure water isreduced to the small gap 220. This arrangement significantly reduces theimpurities from the lens into the immersion liquid and reducesimpairments that may result to a work surface, such as a wafer.

Another approach to protecting a wet lens element from liquiddegradation includes applying a series of coatings to the non-opticalareas of a wet lens element that are exposed to a liquid in an immersionlithography apparatus. FIG. 3 is used to describe this approach.

FIG. 3 is a flowchart of method 300 to protect a wet lens element fromliquid degradation, according to an embodiment of the present invention.The method 300 begins with a step 310. In step 310, a liquidorganoxy-metallic compound is diluted with a thinning agent. Examplethinning agents include, but are not limited to 1-butanol, 99.8%,anhydrous; ethyl acetate, 99.8%, HPLC grade; and dichloromethane, 99.8%,anhydrous. These thinning agents are commonly used and available frommultiple suppliers with identical specifications. Other thinning agentswill be known to persons skilled in the relevant art(s) from theteachings herein.

In one embodiment of the invention, the organoxy-metallic compound istitanium (IV) butoxide, which is generally available from AldrichChemical Company, Milwaukee, Wis. Examples of other organoxy-metalliccompounds that may be used include, but are not limited to, the familiesof titanium (IV) alkoxides which are converted to the metal oxides, suchas titanium dioxide. Other organoxy-metallic compounds that can be usedin connection with the present invention will become apparent to personsskilled in the relevant art(s) from the teachings herein.

A preferred ratio of thinning agent to organoxy-metallic compound isabout one to one. Alternative ratios may be used ranging from a ratio ofabout one part thinning agent to three parts liquid inorganic material,to a ratio of about three parts thinning agent to one part liquidinorganic material. Alternatively, no thinning agent may be used.

In a step 320, the diluted organoxy-metallic compound is applied tonon-optical areas 230 creating a coating. The coating covers non-opticalareas from optical surface 225 to a point on non-optical area 230 thatwill not be immersed in liquid. The coating may range in thickness fromabout one nanometer to about two hundred micrometers.

In one embodiment, the diluted organoxy-metallic compound is manuallyapplied using an optical applicator cloth. Other applicators, such as abrush, sponge, blade, or the like, may be used in either approach andwill become apparent to persons skilled in the relevant art(s) from theteachings herein.

In a step 330, the coating is exposed in ambient air to ultra-violetlight and cured to form the light absorbing coating. Broad bandultra-violet light from a Hg or Xe source may be used. Alternatively,monochromatic ultra-violet light from an excimer laser may also be usedand be effective in curing the coating. For example, the inventors usedthree DYMAX 50 WATT (bulb #35003) fiberoptic ultra-violet curing lampsto cure a titanium dioxide coated area. The fibers on the lamps wereabout one inch from the sample, and the exposure time for the sample wasabout 20 minutes. If a diluting agent is used, the organoxy-metalliccompound should set for a sufficient period of time to allow thediluting agent to evaporate.

The coating must be optically opaque to the ultra-violet wavelengthsthat may damage subsequent coatings as discussed below (e.g., 157 nm,193 nm, and 248 nm). In addition, the coating must be mechanicallyrobust and must withstand routine handling of the optical element. Metaloxide films with the appropriate characteristics of optical absorption,optical transmission, mechanical robustness, and ability to adhere tooptical materials, include, but are not limited to SiO₂, Al₂O₅, ZrO₂,HfO₂, Ta₂O₅, Nb₂O₅, and TiO₂.

In a step 340, the light absorbing coating is covered with a liquidshield coating, also referred to as a water resistant coating. Waterresistant coatings with appropriate characteristics includepolyurethane, ultra-violet polyurethane (e.g., DYMAX 901), siliconcaulks and organic encapsulates. When using polyurethane the thicknessof the light absorbing coating is about 10 microns to 500 microns.Optionally, prior to step 340, the light absorbing coating is cleanedwith methanol or similar substance. Furthermore, optionally, prior toinitial use of a wet element lens with liquid degradation protectionapplied using method 300 in a lithographic application, the wet lenselement is soaked in an immersion liquid to allow initial leaching oforganic material from the polyurethane. In one embodiment the soakperiod is about two days. Following soaking, the leaching decreases toan acceptable level for use within an immersion lithographic apparatus.In step 350, method 300 ends.

Additional steps or enhancements to the above steps known to personsskilled in the relevant art(s) from the teachings herein are alsoencompassed by the present invention.

FIGS. 4A and 4B provide diagrams of a side view and top-down view of wetlens element 110 having a metal oxide coating and liquid shield coating,according to an embodiment of the invention. FIGS. 4A and 4B illustratewet lens element 110 after method 300 has been implemented.Specifically, FIG. 4A shows a metal oxide coating 410 that coats theside portions of wet lens element 110. As discussed above, metal oxidecoating 410 can include a variety of different metal oxide films. Metaloxide films with the appropriate characteristics of optical absorption,optical transmission, mechanical robustness, and ability to adhere tooptical materials, include, but are not limited to SiO₂, Al₂O₅, ZrO₂,HfO₂, Ta₂O₅, Nb₂O₅, and TiO₂.

Placed on top of this coating, per step 340, is liquid shield coating420. As discussed above potential liquid shield coatings include, butare not limited to polyurethane, ultra-violet cure polyurethane, siliconcaulks and other organic encapsulates. FIG. 4B shows a top-down view ofwet lens element 110 that illustrates that metal oxide coating 410 andliquid shield coating 420 wrap completely around the side portions ofwet lens element 110 that will be submersed in an immersion liquid.

FIG. 5 provides a plot 500 of test results showing wet lens elementdegradation in an immersion liquid. FIG. 5 provides the results ofaccelerated dissolution tests that compare the degradation of anuntreated CaF₂ lens, a CaF2 lens with only a TiO₂ coating and a CaF₂lens with both a TiO₂ coating and a liquid shield coating ofpolyurethane that were applied using method 300. The lens were each 1.5inches in diameter and about 0.375 inches thick. The immersion liquidwas water. The plot shows the amount of CaF₂ dissolved along thevertical axis against equivalent days of immersion along the horizontalaxis. The data show that the CaF₂ lens coated with both the metal oxidecoating and liquid shield coating have nearly zero dissolution for asmany as 1200 days. The untreated CaF₂ lens, by contrast has significantdissolution. For example, with no treatment the level of dissolution isabout three times above acceptable levels. Similarly, the TiO₂ coatedlens had significant degradation. Thus, plot 500 illustrates thesignificant degradation improvement of a lens coated with a metal oxidecoating and a liquid shield coating, as per method 300.

An exemplary embodiment of a wet lens element having coated edge sidesaccording to the method described in FIG. 3 has been presented. Thepresent invention is not limited to this example. This example ispresented herein for purposes of illustration, and not limitation.Alternatives (including equivalents, extensions, variations, deviations,etc., of those described herein) will be apparent to persons skilled inthe relevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the present invention.In particular, the invention is not limited to the particular type oflens illustrated herein. The invention can be applied to any type oflens element that has non-optical portions that will be submersed in animmersion liquid.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.

1. A projection optics system within an immersion lithography systemcomprising: a series of optical elements; a wet lens element; a metaloxide coating that covers a side portion of the wet lens element; and apolyurethane liquid shield coating that covers the metal oxide coating.2. The projection optics system of claim 1, wherein the wet lens elementcomprises CaF₂.
 3. The projection optics system of claim 1, wherein themetal oxide coating comprises SiO₂, Al₂O₅, ZrO₂, HfO₂, Ta₂O₅, Nb₂O₅, orTiO₂.
 4. The projection optics system of claim 1, wherein thepolyurethane liquid shield coating comprises ultra violet curepolyurethane.
 5. The projection optics system of claim 1, wherein thepolyurethane liquid shield coating is in the range from 10 to 500microns.
 6. The projection optics system of claim 1, wherein the metaloxide coating and the polyurethane liquid shield coating protect the wetlens element from liquid degradation when the wet lens element issubmerged in an immersion liquid.
 7. The projection optics system ofclaim 1, wherein the metal oxide coating is applied to a thickness ofabout one nanometer to not more than about two hundred micrometers. 8.The projection optics system of claim 1, wherein the metal oxide coatingis formed by exposing an organoxy metallic compound to ultra-violetlight.
 9. The projection optics system of claim 8, wherein the organoxymetallic compound comprises an organo-titanium compound.
 10. Theprojection optics system of claim 1, wherein the metal oxide coatingprovides a light absorbing coating that is opaque to ultra-violetwavelength and the polyurethane liquid shield coating provides a waterresistant coating.
 11. A projection optics system, comprising: a seriesof optical elements; a wet lens element; a metal oxide layer on a sideportion of the wet lens element; and a polyurethane layer on the metaloxide coating.
 12. The projection optics system of claim 11, wherein thewet lens element comprises CaF₂.
 13. The projection optics system ofclaim 11, wherein the metal oxide coating comprises SiO₂, Al₂O₅, ZrO₂,HfO₂, Ta₂O₅, Nb₂O₅, or TiO₂.
 14. The projection optics system of claim11, wherein the polyurethane layer comprises ultra violet curepolyurethane.
 15. The projection optics system of claim 11, wherein thepolyurethane layer has a thickness in the range from 10 to 500 microns.16. The projection optics system of claim 11, wherein the metal oxidelayer and the polyurethane layer protect the wet lens element fromliquid degradation when the wet lens element is submerged in animmersion liquid.
 17. The projection optics system of claim 11, whereinthe metal oxide layer has a thickness of about one nanometer to not morethan about two hundred micrometers.
 18. The projection optics system ofclaim 11, wherein the metal oxide layer is formed by exposing anorganoxy metallic compound to ultra-violet light.
 19. The projectionoptics system of claim 18, wherein the organoxy metallic compoundcomprises an organo-titanium compound.
 20. The projection optics systemof claim 11, wherein the metal oxide layer provides a light absorbingcoating that is opaque to ultra-violet wavelength and the polyurethanelayer provides a water resistant coating.